Canvas window changing size when switching from fullscreen to borderless

[Refrigerator]

I recently got a new laptop with a weirdly sized 1920x1200 monitor and I'm having issues setting up games to record and stream. Ideally, I would like to be able to record in 1080p with no black bars, and I'm fine with playing games at lower res than my native monitor to improve the viewing experience on video. I am currently setting up R6 Siege to stream it, but ran into a strange issue I couldn't find an answer to online.

When in fullscreen, the canvas scales up nicely with the output window and fill it completely. When I switch the game to borderless fullscreen, however, the cavas size gets larger and no longer nicely scales up to the output window.
Log file: https://obsproject.com/logs/rdpfiZzVCksgfpcS
Video of what I'm talking about: https://youtube.com/shorts/ujjnrHTaex8?feature=share

 

[Suslik V]

This game picture had size: 1920x1200, see

06:24:44.928: [game-capture: 'Game Capture 2'] attempting to hook process: RainbowSix.exe
06:24:44.963: [game-capture: 'Game Capture 2'] d3d12_init: device=0x1EB405C63F0
06:24:44.963: [game-capture: 'Game Capture 2'] DXGI_SWAP_CHAIN_DESC:
06:24:44.963:     BufferDesc.Width: 1920
06:24:44.963:     BufferDesc.Height: 1200

this was: 1920x1080, see

06:21:13.742:     BufferDesc.Width: 1920
06:21:13.742:     BufferDesc.Height: 1080

and this was: 1902x1080, see

06:20:38.167:     BufferDesc.Width: 1902
06:20:38.167:     BufferDesc.Height: 1080

Borderless full screen means that the Windows (OS) controls display device and the rendered image fits into the given window (like any simple program or text editor do, with maximum sizes determined by OS). Exclusive full screen, means that the game's rendering engine gets full control over the display device, so can set any color depth, refresh rate and resolution no matter what your desktop had before you run the game.

Edit: internally, some games may render own menu and the game at different resolutions.